This invention relates to a shift control for an automatic transmission, and more particularly to a model-based clutch pressure control for carrying out a power-on downshift.
In general, a motor vehicle automatic transmission includes a number of gear elements and selectively engageable friction elements (referred to herein as clutches) that are controlled to establish one of several forward speed ratios between the transmission input and output shafts. The input shaft is coupled to the vehicle engine through a fluid coupling such as a torque converter, and the output shaft is coupled to the vehicle drive wheels through a differential gearset. Shifting from a currently established speed ratio to new speed ratio involves, in most cases, disengaging a clutch (off-going clutch) associated with the current speed ratio and engaging a clutch (on-coming clutch) associated with the new speed ratio.
Various techniques have been used for electronically controlling the on-coming and off-going clutches during a power-on downshift. For example, the U.S. Pat. Nos. 5,029,494 and 5,070,747 to Lentz et al. disclose a downshift control in which the on-coming clutch is filled in preparation for engagement, while the off-going clutch is controlled in a series of steps including (1) progressively releasing the off-going pressure until the off-going clutch begins to slip, (2) controlling the off-going pressure to achieve a desired input speed profile, and (3) controlling the off-going pressure to hold the input speed substantially at the post-shift speed; and thereafter, engaging the on-coming clutch and dis-engaging the off-going clutch. Similar control techniques are also described in the U.S. Pat. No. 4,653,351 to Downs et al., the U.S. Pat. Nos. 4,796,490 and 5,079,970 to Butts et al.
The above-described controls tend to involve numerous calibrated parameters requiring adaptive adjustment to compensate for variations, and have a relatively limited ability to react to changes in input torque during the shift. In practice, a margin of safety is frequently provided by intentionally overlapping the on-coming and off-going clutches to some degree, at the expense of clutch heating and shift quality.
The present invention is directed to an improved control for an automatic transmission power-on downshift, wherein a dynamic model of the transmission is used to schedule the on-coming and off-going clutch pressures based on the transmission input torque and a desired trajectory of the input shaft during the shift. The shift is initiated with the off-going clutch by using the dynamic model to achieve consistent initiation of clutch slippage, and to conform the input speed to the desired trajectory. When the input speed nears a synchronization speed for the target speed ratio, the dynamic model is used to engage the on-coming clutch at a rate based on the input torque while maintaining the input speed in synchronism with the target speed ratio. As a result, the control responds appropriately to dynamic changes in input torque, the input speed more accurately tracks the desired trajectory, and the shifts are completed at or near synchronism. Additionally, scheduling the clutch pressures based on the dynamic model achieves more consistent shift feel and improved adaptability to different powertrain and vehicle-type configurations while reducing the number of calibrated parameters requiring adaptive correction.